1. Field of the Invention
The present invention relates to an organic EL element.
2. Description of the Related Art
Organic electroluminescent (EL) elements include an organic compound layer between a pair of electrodes. The organic compound layer includes one or more sublayers, including a light-emitting sublayer, an optional charge-injection sublayer, and an optional charge-transport sublayer.
In recent years, research and development of various organic EL elements have advanced. One task to be accomplished in the research and development is an improvement in the luminous efficiency of elements. In order to improve the luminous efficiency of elements, use of phosphorescent material instead of fluorescent material has been investigated. Theoretically, use of phosphorescent material, that is, light emission from a triplet excited state is expected to increase luminous efficiency to approximately three times the luminous efficiency of an element utilizing fluorescent material (singlet).
U.S. Pat. No. 6,893,743 discloses an organic EL element that includes three materials (a host, a phosphorescent material A, and a phosphorescent material B) in a light-emitting layer. This organic EL element utilizes energy transfer between the three materials in the light-emitting layer to improve the luminous efficiency of the element.
Two of the phosphorescent materials of the organic EL element according to Patent Document 1 emit phosphorescence at room temperature. In order to emit the light that is attributed to material B, the amounts of the phosphorescent materials in the light-emitting sublayer must be limited. Because the emission quantum efficiencies of the two phosphorescent materials vary with temperature, the luminescent colors at a high temperature and a low temperature may differ from each other.